The question of whether outer space ever ends is one of the most profound inquiries in cosmology. The universe is the entirety of space and time, known as the cosmos, and asking if it ends questions the very limits of existence. Understanding this boundary, or lack thereof, requires distinguishing between what we can observe and the universe in its totality.
The Observable Universe
The most concrete answer to the size of space lies in the concept of the Observable Universe, which represents the sphere of space centered on Earth from which light has had time to reach us. Although the Big Bang occurred 13.8 billion years ago, the radius is not 13.8 billion light-years. This is because the cosmos is expanding. As light travels toward us, the space it moves through simultaneously stretches, pushing the original source further away.
Due to this metric expansion, the current proper distance to the edge of the Observable Universe is estimated to be about 46.5 billion light-years in every direction. This 93 billion light-year diameter represents the practical limit of what humans can ever hope to see. The edge of this observable region is marked by the Cosmic Microwave Background (CMB) radiation, the afterglow of the universe when it first became transparent. While our view of space ends at this horizon, the universe itself continues far beyond this boundary.
The Concept of Boundaries
The idea of space ending often suggests a physical wall or an ultimate “edge.” However, the universe is not thought to possess a boundary in this traditional sense because it is space itself, not an object contained within a larger volume. Asking what lies “outside” the universe loses meaning, similar to asking what is north of the North Pole, because “outside” implies more space.
A more useful concept is that of an unbounded space, meaning one could travel perpetually in a straight line without ever encountering a barrier. This is analogous to the surface of the Earth, which is finite in area yet has no edge. If the universe were like the surface of a sphere, a traveler would eventually return to their starting point, making the space finite but unbounded. The physical space of the cosmos is thought to be unbounded, but whether the total volume is finite or infinite depends on its overall shape.
How Expansion Affects Size
The ultimate size of the universe is constantly influenced by metric expansion, the process where space itself stretches. This expansion is not matter spreading through pre-existing space; it is the creation of new space between galaxies, summarized by Hubble’s Law. The expansion is also accelerating, a discovery attributed to Dark Energy.
Dark Energy acts as a negative pressure that overcomes gravity on large scales, driving this continuous growth. This means the volume of the universe increases over time, regardless of whether its total size is finite or infinite. The effect of this expansion is that distant galaxies eventually recede faster than the speed of light because the space between us stretches so rapidly. While this does not define an end to space, it sets an event horizon beyond which light from these isolated galaxies will never reach us.
The Geometry of the Universe
The theoretical answer to whether space truly ends lies in the global geometry of the universe, which is determined by the total density of matter and energy. This relationship is quantified by the density parameter, Omega (\(\Omega\)), which compares the universe’s actual density to the critical density required for a flat geometry. There are three possibilities for the overall shape of the cosmos:
- If the density is greater than the critical density (\(\Omega > 1\)), the universe has positive curvature, making it a closed space. This geometry is finite in volume but unbounded, meaning a traveler would eventually loop back to their start.
- If the density is less than the critical density (\(\Omega < 1[/latex]), the universe has negative curvature, resulting in an open and infinite space.
- If the density exactly equals the critical density ([latex]\Omega = 1\)), the result is a flat geometry, similar to a vast, infinite plane.
Precise measurements of the Cosmic Microwave Background, particularly from missions like WMAP and Planck, show that the universe’s geometry is remarkably close to flat. The total density parameter is measured to be \(1.00\) within a small margin of error. This observational evidence strongly suggests that the universe is spatially infinite and, therefore, does not have an end.